In a dual screen setup, new xfce windows or dialogs are not
always appearing on the primary screen.
Depending on the window or dialog, it appears on the primary
screen or on the other screen.
This behaviour appeared after upgrade to Debian 9, it was working
properly in Debian 8.
Note that I tried by replacing xfxm4 by metacity. At least the weather widget
test (see below) then works properly.

For example, when clicking on <user> -> Log Out, the dialog appears on the
primary screen, while when clicking on <user> -> Shutdown,
the shutdown confirmation dialog appears on the other screen.

Here are some applications that appear on the primary screen:
xfce4-terminal
xfce4-mime-settings
xfce4-clipman-settings
xfce4-session-settings
xfce4-notes-settings

Here are some applications that appear on the other screen:
xfce4-accessibility-settings
xfce4-appearance-settings
xfce4-display-settings
xfce4-keyboard-settings
xfce4-mouse-settings
xfce4-power-manager-settings
xfce4-sensors


If I click on the weather widget (which is in a panel on the
primary screen), the detailed weather report is opened on
the other screen.

nvidia card GeForce GTX 750 Ti, with nvidia driver 375.82

xrandr properly indicates the primary screen:
    $ xrandr
    Screen 0: minimum 8 x 8, current 3200 x 1080, maximum 16384 x 16384
    VGA-0 connected 1280x1024+0+0 (normal left inverted right x axis y axis)
360mm x 270mm
       720x400       85.04 +
       1600x1200     75.00
       1280x1024     85.02*   75.02
       1280x960      85.00
       1152x864      75.00
       1024x768     100.00    85.00    75.03    60.00
       800x600       85.06    75.00
       640x480       85.01    75.00    59.94
    DVI-D-0 disconnected (normal left inverted right x axis y axis)
    HDMI-0 connected primary 1920x1080+1280+0 (normal left inverted right x
axis y axis) 521mm x 293mm
       1920x1080     60.00*+  59.94    50.00    60.00    50.04
       1680x1050     59.95
       1440x900      74.98    59.89
       1280x1024     75.02    60.02
       1280x720      60.00    59.94    50.00
       1024x768      75.03    60.00
       800x600       75.00    60.32
       720x576       50.00
       720x480       59.94
       640x480       75.00    72.81    59.94    59.93

In Window Manager Tweaks Placement, I have tried all possible settings,
but none of the settings are influencing the positionning of applications
appearing on the other screen.
The list above was done with the setting:
   Minimum size to trigger smart placement : Small
   place windows Under the mouse pointer

Note that e.g. when I am setting
   Minimum size to trigger smart placement : Large
   place windows At the centre of the screen
then e.g. xfce4-terminal will appear at the centre of the screen.

So, it looks like some dialogs/widgets/screens are respecting the window
manager positionning, while others are not.

Thanks

(note: I have reported this via Debian bug first, 
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=876547
Seeing no activity there, I am trying upstream ...

-- System Information:
Debian Release: 9.1
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.9.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8),
LANGUAGE=en_US:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages xfce4 depends on:
ii  gtk2-engines-xfce        3.2.0-2
ii  libxfce4ui-utils         4.12.1-2
ii  orage                    4.12.1-3
ii  thunar                   1.6.11-1
ii  xfce4-appfinder          4.12.0-2
ii  xfce4-panel              4.12.1-2
ii  xfce4-pulseaudio-plugin  0.2.4-1
ii  xfce4-session            4.12.1-5
ii  xfce4-settings           4.12.1-1
ii  xfconf                   4.12.1-1
ii  xfdesktop4               4.12.3-3
ii  xfwm4                    4.12.4-1